The key is to support the sink while you tighten.
How to unmount a wall mounted sink.
Use a utility knife to sever any caulking between the back edge of the sink and the wall if applicable to ease removal.
When there s no wiggle left between the sink and the wall you re done.
If your sink is overmounted the removal process is relatively easy though depending on the size and weight of the sink you may want to get a friend to help.
If your sink s flange is secured to the granite with glue or a similar substance you ll need to remove it but it s likely that you need only lift the sink up and out of its place in the granite countertop.
How to attach an undermount sink on a stone countertop.
Remove the sink from its wall mounting bracket by lifting evenly upward approximately 3 inches from both sides of the sink near the wall.
Turn off the water again.